Generally, there are two types of radars that are used in a civil Air Traffic Control (ATC) radar system: a Primary Surveillance Radar (PSR) and a Secondary Surveillance Radar (SSR). The PSR transmits pulses and reports the range and azimuth of all detected objects in a given surveillance area. The detected objects include both aircraft and non-aircraft objects. The SSR transmits interrogation signals to aircrafts in the given surveillance area and receives information from the aircrafts that have operational transponders that respond to the interrogation signals. The information includes the range, azimuth, identity and height of all aircrafts that reply to the interrogation signals.
However, some aircrafts, such as hijacked or enemy aircrafts, may deliberately turn off their transponders. Other aircrafts may have a damaged transponder. Furthermore, non-aircraft airborne objects, such as birds, cannot respond to the interrogation signals. As a result conventional civil ATC radar systems cannot determine the height of these non-aircraft objects or aircrafts that do not respond to the SSR interrogation pulses, which can be a serious problem. For example, in the United States, there were over 6,000 reported collisions between aircrafts and birds in 2004. Most of these collisions occurred near airports at low elevations in the glide path where aircrafts were either landing or taking off. This is also the area in which aircraft are most vulnerable to collisions.
Furthermore, in the past decade many countries, including the UK, the Netherlands, Germany and the USA have launched programs to deploy wind turbines as an alternative, environmentally friendly source of electrical energy. However, this has raised many concerns from ATC and military authorities since radar returns from wind turbines have the potential to distract and confuse air traffic controllers and can effectively mask genuine aircraft returns in the vicinity of the wind farm.
In fact, the presence of wind farms within the field of view of primary surveillance radars presents a considerable design challenge. Echoes originating from these structures may have similar characteristics to those of an aircraft and may be significantly stronger in amplitude. The overall effect of wind turbines are three fold: 1) the echoes (i.e. radar return signals) due to wind turbines may dominate and mask those originating from an aircraft resulting in a “radar blind zone” and missed detections, 2) the aircraft track may be seduced away from its correct path due to miss-association with an echo originating from a wind turbine, and 3) echoes originating from a wind farm may result in the generation of a high rate of false reports in the vicinity of the wind farm.
For instance, very large wind turbines have a Radar Cross Section (RCS) of up to 25 dBsm on average and in some cases even as high as 50 dBsm, whereas the typical RCS of a commercial aircraft during approach (i.e. when landing) ranges from 3 dBsm to 10 dBsm. In addition, the Doppler frequencies of the radar returns from the rotating blades of a wind turbine are similar to the Doppler frequencies of an approaching aircraft (an example is 1671 Hz at a frequency of 2,800 MHz which corresponds to a velocity of 174 knots while the approach speed of a commercial aircraft is about 150 knots). Thus, radar returns from wind turbines have similar Doppler characteristics and larger RCS than aircraft and can completely mask a radar return from an aircraft virtually making it “invisible” to a radar system when in the vicinity of a wind farm. In fact, wind farm regions result in a 16% to 22% lower probability of detection of aircraft by a civil ATC radar system than in adjacent non-wind farm regions.